The present invention relates generally to delay equalizers, also called all-pass filters, which are a class of networks exhibiting a flat frequency response but introducing prescribed phase shift versus frequency. The invention more particularly relates to delay equalizer sections which can be the building blocks of a larger delay network. The delay sections of interest generally have second order all-pass transfer functions, are tunable, and have particular application in compensating for discovered phase anomalies in digital audio systems, such as digital tape recorders. The invention also relates to the correction of the discovered phase anomalies in digital audio systems.
Digital audio systems generally require very steep filtering at about 20 KHz to prevent a phenomenon known as "aliasing" of the systems audio signal with the system's sampling frequency, now standardized at 44 KHz. To achieve steep attenuation near 20 KHz multi-pole filters have been devised known as anti-aliasing filters. Conventionally, such filters contain 13 to 24 poles and are difficult to build. While successfully achieving a satisfactory amplitude roll-off, it has been discovered that the anti-aliasing filter introduces phase distortion to the system and that such distortion detrimentally affects the system's audio performance.
To analyze the phase or time delay distortion in an audio system it is necessary to characterize the phase response of the system. In addition to pure phase shift, two different parameters are commonly used to define system phase response: One is phase delay (t.sub.p) and the other is group delay (t.sub.g). Phase delay and group delay are given by the formulas: EQU t.sub.p =-p/2.pi.f EQU t.sub.g =(-1/2.pi.)+(dp/df)
where p is the phase relationship between input and output signal and f is the frequency.
Conceptually, group delay represents the time that each frequency is delayed compared to other frequencies passed through the system. Stated differently, group delay will define how well an impulse (or any burst of frequencies) will be preserved as it passes through the system. Passing an impulse through a system which has constant (i.e. linear) group delay will not alter the pulse. Thus, any pure time delay (i.e. constant or linear time delay) however large will not alter the shape of the impulse, it will only delay it in time; non-linear group delay on the other hand will cause pulse degradation.
In this specification the term "time delay" will be used interchangeably with "group delay" since as used herein both are analogous. It will be understood, however, that there are conditions and circumstances where this analogy cannot be easily made, but such exceptions are not important to this disclosure. Because "time delay" is the more commonly used term in the audio industry and because when linear, time delay can be measured (group delay is calculated from phase response) "time delay" will normally be referred to.
Referring now to our digital system, the system will exhibit a total time delay which is the product of two introduced phase components: a linear, pure time delay, and a non-linear, frequency dependent time delay. The pure delay component results primarily from the data conversion process and time base correction for the recording medium; the system anti-aliasing filter and output smoothing filter also contribute a small amount of pure delay. However, the non-linear delay component which is believed to detrimentally affect the audio response of the digital system is contributed primarily by the anti-aliasing filter. (The output smoothing filter of the digital system also contributes a small amount of non-linear delay.) Non-linear delay can be measured in a digital system by subtracting the linear delay components. This is done by comparing the output of the digital system with a reference signal consisting of the original signal suitably delayed by a high quality delay line to reproduce the linear delay component of the digital system. A test apparatus for achieving this measurement is shown in FIG. 1. Using an FFT Analyzer a digital recorder phase response was measured and is shown in FIG. 2 of the drawings. From this phase response the group delay characteristic of the digital recorder was calculated from the above formulas and this characteristic is shown in FIG. 3.
With this discovered phenomenon in digital systems the problem is how to overcome the resultant degradation of the audio output. It has been discovered that improved performance can be achieved by adding time delay to the overall digital system at the lower and mid range frequencies such that the time delay of the digital system over its operating frequency range, near DC to 20 KHz, will be relatively frequency independent. Thus, the invention in one aspect involves means for delay equalization which adds delay from near DC to where the group delay curve of a digital system begins to increase with frequency (See FIG. 3), and then, where the group delay of the system is increasing, adding decreasing amounts of time delay to provide a composite relatively flat delay curve versus frequency.
The difficulty of implementing such a delay equalization is that a suitable delay equalization network would require numerous poles and would have to be precisely tuned to achieve a desired equalization. While theoretically such a circuit could be devised, in practice it would be quite difficult, since conventional delay equalizers do not have the capability of being easily tuned and require high precision parts. In the present invention, an active delay equalizer section has been devised which is easily tunable, which has separately tunable circuit parameters, and which can be readily and non-interactively cascaded with other sections. A multipole delay equalizer is provided which can be constructed with relatively low tolerance parts and which can be readily trimmed for a desired delay characteristic.